Not applicable.
This invention relates to inhibitors, compositions, and processes for deoxidizing and cleaning surfaces of aluminum and its alloys that contain at least 55% by weight of aluminum (all such alloys being hereinafter to be understood as encompassed within the scope of the term xe2x80x9caluminumxe2x80x9d unless the context requires otherwise), while minimizing the etching of titanium. xe2x80x9cDeoxidizingxe2x80x9d is to be understood herein as the removal from the surface of metals of oxide films and other adherent inorganic materials that would reduce adhesion to subsequently applied protective coatings such as conversion coatings and/or paints and the like, and xe2x80x9ccleaningxe2x80x9d means removal of all other foreign materials, especially organic soils and poorly adherent inorganic substances such as metal dust and the like, that would reduce adhesion to such subsequently applied protective coatings.
With most deoxidizing agents, especially acidic aqueous liquid compositions with substantial hexavalent chromium and/or free fluoride ion contents, such compositions being probably the most effective chemical classes of cleaners and deoxidizers for aluminum now known, there is a perceptible but controlled etching or dissolution of the aluminum, from its surface inward, while the deoxidizing agent is in contact with it. In the aerospace industry in particular, such deoxidizing is considered a necessity for achieving adequate corrosion resistance for many uses of aluminum. The aluminum parts being deoxidized are often sup ported by racks or other structures of titanium during the deoxidizing process, and etching of these titanium structures during the deoxidizing of aluminum is very disadvantageous, as it decreases the lifetime of the titanium structures and would necessitate frequent replacement of these expensive items.
Hexavalent chromium-containing deoxidizing liquid compositions for the types of aluminum alloys most commonly used in aerospace have low etch rates for titanium. However, the pollution problems associated with hexavalent chromium have motivated efforts to eliminate or reduce its use as much as possible. Most previously developed chromium-free deoxidizers for aluminum, however, have had unsatisfactorily high etch rates on titanium and/or have required an additional process step compared with conventional deoxidizing of aluminum with the use of hexavalent chromium containing deoxidizing liquid compositions, thereby making them unacceptable to most commercial users.
U.S. Pat. No. 5,637,252 of Jun. 10, 1997 to Johnson et al. teaches chromium-free deoxidizing compositions that have acceptably low etch rates for titanium. However, in commercial practice these compositions have sometimes been found objectionable because of one or more of: foaming when used; poorer salt spray test results on the aluminum substrates deoxidized with them, either immediately or after subsequent conventional corrosion resistant surface treatments; and imperfect wetting, as indicated by the presence of xe2x80x9cwater breaksxe2x80x9d, i.e., areas of thicknesses of a coating of water that are visibly different from one point to another on the surfaces of the deoxidized aluminum after using these new deoxidizing compositions.
Accordingly, a major object of the invention is to provide compositions and processes for deoxidizing and cleaning aluminum surfaces that also achieve at least one of the following: consistently low corrosion of the aluminum surfaces deoxidized; absence of foaming during use to an extent sufficient to cause operating difficulties; and uniform wetting by water of the deoxidized surface. Another alternative or concurrent object is to reduce pollution potential from aluminum deoxidizing compositions that contain chromium, ferricyanide, or other polluting constituents. Other alternative or concurrent general and/or detailed objects will be apparent from the description below.
Except in the claims and the operating examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word xe2x80x9caboutxe2x80x9d in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred, however. Also, throughout the description, unless expressly stated to the contrary: percent, xe2x80x9cparts ofxe2x80x9d, and ratio values are by weight or mass; the term xe2x80x9cpolymerxe2x80x9d includes xe2x80x9coligomerxe2x80x9d, xe2x80x9ccopolymerxe2x80x9d, xe2x80x9cterpolymerxe2x80x9d and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description or of generation in situ within the composition by chemical reaction(s) noted in the specification between one or more newly added constituents and one or more constituents already present in the composition when the other constituents are added, and does not preclude unspecified chemical interactions among the constituents of a mixture once mixed; specification of constituents in ionic form additionally implies the presence of sufficient counterions to produce electrical neutrality for the composition as a whole and for any substance added to the composition; any counterions thus implicitly specified preferably are selected from among other constituents explicitly specified in ionic form, to the extent possible; otherwise such counterions may be freely selected, except for avoiding counterions that act adversely to an object of the invention; the word xe2x80x9cmolexe2x80x9d means xe2x80x9cgram molexe2x80x9d, and the word itself and all of its grammatical variations may be used for any chemical species defined by all of the types and numbers of atoms present in it, irrespective of whether the species is ionic, neutral, unstable, hypothetical, or in fact a stable neutral substance with well defined molecules; the terms xe2x80x9csolutionxe2x80x9d, xe2x80x9csolublexe2x80x9d, xe2x80x9chomogeneousxe2x80x9d, and the like are to be understood as including not only true equilibrium solutions or homogeneity but also dispersions that show no visually detectable tendency toward phase separation over a period of observation of at least 100, or preferably at least 1000, hours during which the material is mechanically undisturbed and the temperature of the material is maintained within the range of 18-25xc2x0 C.; and the first definition of an acronym or other abbreviation applies to all subsequent uses of the same acronym or other abbreviation.
It has been found that a combination of fluoroboric acid, a stronger acid, an oxidizing agent, and at least one azole inhibitor, provides a fully acceptable rate and quality of deoxidizing aluminum and achieves at least one other object of this invention as noted above.